With the explosive increase in demand for a broadband multimedia communication service such as the Internet or a high-definition digital TV broadcast, a dense wavelength-division multiplexing optical fiber communication system, which is suitable for a long-distance and large-capacity transmission and is highly reliable, has been introduced in trunk line networks and metropolitan area networks. In access networks, an optical fiber access service spreads rapidly. In such an optical fiber communication system, cost reduction for laying optical fibers as optical transmission lines and improvement of spectral efficiency per optical fiber are important. Therefore, a wavelength-division multiplexing technology which multiplexes multiple optical signals having different wavelengths is widely used.
In an optical transmitter for a wavelength-division multiplexing communication system, an optical modulator is required. In the optical modulator, high-speed optical modulation with small wavelength dependence is indispensable. Further, an unwanted optical phase modulation component which degrades the waveform of the received optical signal after long-distance transmission (in the case of using optical intensity modulation as a modulation method), or an optical intensity modulation component (in the case of using optical phase modulation as a modulation method) should be suppressed to be as small as possible. A Mach-Zehnder (MZ) optical intensity modulator in which waveguide-type optical phase modulators are incorporated into a pair of delay paths in an optical waveguide-type MZ interferometer is suitable for such a use.
In a currently used MZ optical intensity modulator, lithium niobate (LiNbO3, hereinafter referred to as “LN”), which is a typical electro-optic crystal having a Pockels effect in which the refractive index linearly changes with respect to an applied electric field, is used as a substrate. This MZ optical intensity modulator is based on a Ti-diffused optical waveguide circuit of a so-called planar structure that uses the phenomenon in which a region obtained by diffusing titanium (Ti) deposited on the surface thereof into the substrate at high temperature has a high refractive index. As a specific configuration of the planar optical waveguide circuit, a configuration is generally employed in which a pair of a waveguide-type optical phase modulator and a waveguide-type optical multiplexer/demultiplexer is monolithically integrated on the LN substrate to from an MZ interferometer, and electrodes for applying an electric field to this waveguide-type optical phase modulator are provided.
In addition, attempts have been actively made to develop an optical waveguide-type semiconductor optical phase modulator and a semiconductor MZ optical modulator by using a III-V compound semiconductor, such as gallium arsenide (GaAs) or indium phosphide (InP), which is a direct transition type semiconductor useful for integration of a light source element. The optical phase modulator regions of these modulators are formed of a single-mode optical waveguide having a p-i-n type diode structure of a double heterostructure. In this configuration, a high refractive index layer (core layer) that is undoped and has low bandgap energy is formed of a medium (for example, multiple mixed crystal of a III-V compound semiconductor, or a layered structure based on this) whose (complex) refractive index with respect to a optical signal to be modulated changes depending on the field strength. The high refractive index layer (core layer) is sandwiched between low refractive index layers (cladding layers) which have p-type and n-type conductivities, respectively, and are formed of semiconductor having bandgap energy higher than that of the high refractive index layer (core layer). An electric field necessary for optical modulation is generated by applying a reverse bias voltage to the p-i-n type diode structure.
As for the optical modulation efficiency of these MZ optical modulators, in general, when the modulation frequency is in the vicinity of a direct current, the wavelength of a modulation electric signal is substantially in reverse proportion to the length of each optical phase modulator region. Accordingly, it is advantageous to increase the length of each optical phase modulator region so as to reduce a driving voltage amplitude. However, when the modulation frequency is high and the length of each optical phase modulator region is equal to or greater than the propagating wavelength of the modulation electric signal, a modulation electric signal distribution along an optical signal propagation axis of each optical phase modulator region cannot be regarded as being uniform. For this reason, the optical modulation efficiency (or driving voltage amplitude) does not satisfy the inversely proportional relationship with the length of each optical phase modulator region. As the length of an optical phase modulator region increases, the capacitance increases, which hinders improvement in modulation bandwidth. To solve such a problem inherent in increasing the length of the modulator so as to reduce the driving voltage, a so-called travelling-wave electrode is generally used. In the travelling-wave electrode, each optical phase modulator region is regarded as being a transmission line, and a modulation electric signal to be applied to the transmission line is regarded as being a travelling wave. Further, contrivance is made to approximate the phase velocities so that the interaction length between the modulation electric signal and the optical signal to be modulated becomes as long as possible. The optical modulator having such a travelling-wave electrode structure is commercially available and widely used as a key component of an optical transmitter for a long-distance, large-capacity optical fiber communication system of 2.5 to 40 Gb/s.
It is considered that the demand for increasing the transmission capacity for an optical fiber communication system has been increasing, along with an increasing demand for communication. However, if the capacity for this system is increased by continuously increasing only the modulation speed of the conventional binary digital optical intensity modulation without changing the optical fiber serving as a transmission path, it is inevitable that the transmission speed will increase and the optical wavelength at the reception end significantly deteriorate. This is because even when an ideal travelling-wave optical modulator with sufficiently high speed and suppressed wavelength chirping is used, the modulator is affected by the dispersion in the optical fiber and the non-linear effect. For this reason, it is not practical, in view of expandability, that an increase in transmission capacity is required only for high-speed modulation.
It is anticipated that such a problem can be solved and the transmission distance and bandwidth utilization efficiency can be further increased, by applying, to optical communications, complex amplitude multilevel or subcarrier multiplexing techniques, such as a quadrature amplitude modulation (hereinafter “QAM”) system and an orthogonal frequency division multiplex (hereinafter “OFDM”) modulation system, which have first been put into practical use in the field of radio communications. In these optical modulation systems, a combination of an amplitude and a phase (or a real part and an imaginary part) of an optical signal is correlated with multiple bits of modulation data, and an optical modulation signal becomes a complex optical modulation signal. That is, increasing the spectral efficiency of an optical modulation code means setting the level of each of the amplitude and the phase of the optical signal to multilevel values.
To generate such a complex optical modulation signal by using the above-mentioned travelling-wave optical modulator, the modulation electric signal to be applied thereto needs to be changed from a binary digital signal to a multilevel pseudo analog signal. When an analog electric signal having an arbitrary amplitude is generated in a pseudo manner, a digital-to-analog converter (hereinafter “DAC”) that satisfies a set resolution (the number of bits) depending on the grayscale and a required settling time has been widely used. However, only a conversion rate of about several GHz is obtained in the search and development stages, though the conversion rate depends on the internal circuit configuration of the DAC and the like. As the settling time becomes shorter, the set resolution of the output amplitude tends to be lower (the number of bits of the converted output decreases). At present, the resolution of the DAC that can achieve a settling time of about several 100 psec is 4 to 6 bits (16 to 64 grayscales) at most. This is not sufficient for application to a multilevel/multiplexing optical modulator in view of discretization errors.
In addition, 1 Vp-p or more of a maximum voltage (or maximum current) amplitude to be output can hardly be expected to be adaptable to generation of a pseudo analog waveform of several GHz. It is necessary to provide a driving circuit (integrated circuit) for linearly amplifying the analog electric signal output from the DAC with as small a distortion as possible to obtain a sufficient voltage amplitude (at present, about 3.3 to 7 V) for driving the optical modulator. However, also in this case, it is not easy to truly amplify the analog electric signal linearly, which varies at a high rate of several tens of GHz, even if the characteristics of the amplification element itself are improved and the circuit is devised.
It is considered that such a problem inherent in the generation of the multilevel/multiplexing optical modulation signal by use of the optical modulator having a travelling-wave electrode structure is caused due to the fact that there is only one excitation point at which the modulation electric signal is applied to the travelling-wave electrode. Therefore, to solve such a problem, it is necessary to review the existing travelling-wave optical modulator that drives each optical phase modulator region from only one excitation point, and to develop a new structure and a drive system suitable for the new structure. As a specific example thereof, a linear accelerator-type column electrode structure optical modulator is proposed (Non Patent Literatures 1 and 2). The linear accelerator-type column electrode structure optical modulator has a configuration in which a plurality of short optical phase modulators is connected in cascade and dedicated individual driving circuits are provided for each optical phase modulator. The individual driving circuits are sequentially driven at a timing when a optical signal to be modulated sequentially passes through the row of optical phase modulators.
It is expected that the linear accelerator-type column electrode structure can effectively solve the above-mentioned problem inherent in the travelling-wave optical modulator. However, to fully exploit the performance of this structure, as described above, it is necessary to provide an individual driving circuit for each of the short optical phase modulators, and the configuration in which the individual driving circuits are sequentially driven in synchronization with the timing when the optical to be modulated signal sequentially passes through the optical phase modulators connected in cascade. In consideration of such conditions, it is required that a variation in synchronization signal input of each of the individual driving circuits and a variation in propagation delay of the driving signal output be suppressed to the picosecond (psec) order or less, though the variations depend on the number of stages connected in cascade and the length of each optical phase modulation region.
Accordingly, in terms of the control for the propagation time in electrical lines, an integrated circuit in which a plurality of individual driving circuits is monolithically integrated on a semiconductor substrate is desirably used as a drive IC exclusively used for the linear accelerator-type column electrode structure. Further, a routing line to each optical modulator from a drive output signal terminal of the integrated circuit is desirably as short as possible. For the reasons regarding the propagation time control as described above, the integrated circuit that is disposed in the direction parallel to the propagation axis of the optical signal to be modulated, i.e., disposed in the vicinity of the long side of the optical modulator, can have the most practical configuration. Such a layout example of the integrated circuit has already been proposed (Patent Literature 1). At present, an InP-based MZ optical intensity modulator having such a linear accelerator-type column electrode structure and an integrated circuit manufactured by a leading CMOS process have been produced experimentally and continuously reviewed to demonstrate the possibility thereof.
A digital segmented electrode structure multilevel optical modulator module 200 disclosed in Patent Literature 1 will be described below. FIG. 3 is a block diagram of the digital segmented electrode structure multilevel optical modulator module 200 according to Patent Literature 1. As shown in FIG. 3, the digital segmented electrode structure multilevel optical modulator module 200 includes a digital segmented electrode structure optical modulator 201 and integrated circuits 202a and 202b. 
The digital segmented electrode structure optical modulator 201 has an MZ interferometer structure including two single-mode semiconductor optical waveguides 211 and a two-input/two-output optical multiplexer/demultiplexer 212. As shown in FIG. 3, a optical signal to be modulated Input is input from the left side, and an output signal Output and a monitor output Monitor are output from the right side. Each of the two semiconductor optical waveguides 211 serving as a pair of delay paths in the MZ interferometer is provided with a digital segmented electrode structure optical phase modulator 213.
Each of the semiconductor optical waveguides 211 includes a core layer and cladding layers that sandwich the core layer. In each of the semiconductor optical waveguides 211, an electric field can be applied to the core layer, or an electric current can be injected into the core layer, thereby enabling change of its refractive index which affects the optical signal propagating along the core layer. In each of the semiconductor optical waveguides 211, a laterally tapered spot size converter (not shown) is provided in the vicinity of both cleaved end faces of the digital segmented electrode structure optical modulator 1, and low reflection films (not shown) are formed at both cleaved faces.
The digital segmented electrode structure optical phase modulator 213 is segmented into n (n>2, n is an integer) waveguide-type optical phase modulator regions 214 to define small segments of the semiconductor optical waveguides 211. For example, the digital segmented electrode structure optical phase modulator 213 can be segmented into a power-of-two number, i.e., n=2h (h≧2, h is an integer), of waveguide-type optical phase modulator regions 214. FIG. 3 shows an example assuming h=3. The adjacent waveguide-type optical phase modulator regions 214 are electrically isolated from each other.
Each of the integrated circuits 202a and 202b includes m (m≦n, m is an integer) individual driving circuits 221 and m terminators 222. The configurations of the integrated circuits 202a and 202b have a mirror image relationship with respect to the digital segmented electrode structure optical modulator 201. FIG. 3 shows an example assuming m=(2h−1). Each of the individual driving circuits 221 is a circuit block including a branch 223, a driving circuit 224, and a phase shift circuit 225. The branch 223 is a one-input/two-output branch that divides an input clock signal CLK into two signals.
The driving circuit 224 outputs discriminated digital input signals D1 to D7 to the respective waveguide-type optical phase modulator regions 214 in synchronization with one of the divided clock signals CLK. An output stage of the driving circuit 224 has function of delaying, amplitude adjustment, bias adjustment, and waveform shaping. These functions can be controlled by external electric signals (signals C1 to C7 shown in FIG. 3). The functions of the driving circuit 224 can be implemented by applying a D-flip-flop circuit (D-FF circuit) as shown in FIG. 3, for example.
The phase shift circuit 225 outputs the other of the divided clock signals CLK to the subsequent-stage individual driving circuit 221. Similarly, the phase shift circuit 225 has functions of delay adjustment, amplitude adjustment, and waveform shaping. These functions can be controlled by external electric signals as in the driving circuit 224.
An offset signal Offset for adjusting an offset in the phase of the optical signal to be modulated is input to the first waveguide-type optical phase modulator region 214 counted from the input side. The signal output of the i-th (2≦i≦m=2h−1, i is a natural number) individual driving circuit 221 counted from the input side and the (i+1)-th waveguide-type optical phase modulator region 214 are connected by a driving signal line 203.
A terminator 226 that terminates the clock signal transmitted through each of the individual driving circuits 221 is connected between a ground potential and the last-stage individual driving circuit 221 counted from the input side. A terminator 222 is connected between the corresponding driving signal line 203 and a common ground (not shown) to suppress a waveform distortion or degradation in bandwidth due to reflection of the signal output.
Semiconductor optical waveguides 211a that smoothly connect the optical multiplexer/demultiplexer 212 with the waveguide-type optical phase modulator regions 214 adjacent to the optical multiplexer/demultiplexer 212 are connected to potential clamp means 204 of a potential VFIX. This allows the optical multiplexer/demultiplexer 212 and the semiconductor optical waveguides 211a to be connected to an external constant voltage source and kept at a constant potential regardless of the magnitude of the driving signal.
Next, the operation of the digital segmented electrode structure multilevel optical modulator module 200 will be described. The clock signal CLK input to the digital segmented electrode structure multilevel optical modulator module 200 is first divided into two parts at the branch 223. One of the divided clock signals CLK is guided to a clock signal input of each of the individual driving circuits 221. The individual driving circuits 221 discriminate the logic of each of the digital input signals D1 to D7 in synchronization with the divided clock signals CLK, and drive the waveguide-type optical phase modulator regions 214 according to the result.
The other of the divided clock signals CLK is guided to the subsequent-stage individual driving circuit 221 through the phase shift circuit 225. By repeating this, the (2h−1) individual driving circuits 221 can sequentially drive the respectively connected waveguide-type optical phase modulator regions 214.
Thus, when the modulated signal Input is input from the left side, the digital segmented electrode structure multilevel optical modulator module 200 can modulate the optical signal to be modulated.